This invention is an improvement of the Automatic Pellet Producing System of U.S. Pat. No. 3,932,736, the entirety of which is incorporated herein by reference. Essentially, that patent discloses an automatic pellet producing system wherein the moisture content of the material at the mill is automatically controlled to within a prescribed range, and the feed rate of the material to the mill is automatically controlled in response to the moisture content control. In a preferred embodiment of that patent, the moisture content is controlled by sensing a temperature difference (.DELTA.T) which is the difference between the temperature of the material at a first location of the material prior to entering the mill, and the temperature of the material at a second location as it enters the mill. The referenced patent also includes, among other features, a load control system for automatically controlling the load on the pellet mill when the moisture control conditions are satisfied.
The present invention represents an improvement over the cited patent in the selection of .DELTA.T. As explained, the .DELTA.T of the referenced patent is measured by sensing the temperature of the material at first and second locations prior to entering the mill.
In accordance with the present invention, .DELTA.T is measured by sensing temperatures of the material across the mill (.DELTA.T mill), or a parameter that is directly related to the .DELTA.T across the mill.
One of the problems in producing pellets has been to control the moisure content of the material to be pelleted. For a given formulation, for proper pellet consistency, minimum load on the mill, and maximum production, are interrelated. It has been discovered that .DELTA.T mill is very closely, if not directly, related to the work done by the mill in producing the pellets. The greater .DELTA.T mill, the greater the work, and conversely the lower .DELTA.T mill, the less the work done by the mill in producing the pellets. It has also been discovered that for a given feed formulation there is a range of .DELTA.T mill that will give good pellet quality, keep the work done by the mill to a minimum, and allow maximum pellet production.
One of the advantages of the present invention is that the use of .DELTA.T mill takes into account the peculiarities inherent within the mill itself. Although the specifications of mills of the same type, load capacity, etc., may appear to be the same, there are always inherent characteristics peculiar to each machine. For example, when comparing supposedly identical machines, with both operating at the same work level and at the same production speed, the load on one may be less than the other. By operating both machines within a prescribed .DELTA.T mill range, the production rate of each can be adjusted for maximum production within the prescribed load limitations.
Generally, in accordance with the present invention, a prescribed .DELTA.T mill range is selected. .DELTA.T mill is then sensed and the proportion of certain ingredients in the formulation is controlled to maintain .DELTA.T mill within the prescribed range.
In a preferred embodiment of the invention, .DELTA.T mill is sensed by sensing the temperature of the material as it enters the mill, sensing the temperature of the material at the output of the mill, and measuring the difference between these temperatures (.DELTA.T mill). The amount of moisture in the material is controlled to maintain .DELTA.T mill within the selected range. The moisture content may be controlled by controlling the amount of steam, water, dry heat, or some other suitable moisture additive or moisture controlling ingredient. The more moisture in the material, the lower the .DELTA.T mill. The less amount of moisture, the greater the .DELTA.T mill. Upon satisfying the .DELTA.T mill condition, the feed rate of the material into the mill is controlled for maximum productivity within the load limitations of the mill.